3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal
Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. A wireless device such as a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNodeB) in an E-UTRAN and as a NodeB in UTRAN. An RBS is a general term for a radio network node capable of transmitting radio signals to the UE and receiving signals transmitted by the UE. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB.
FIG. 1 illustrates a conventional radio access network in an LTE system. An eNodeB 101a with an antenna or transmission point 102a serves a UE 103 located within the eNodeB's geographical area of service also called a cell 105a. The eNodeB 101a is directly connected to a core network (not illustrated). The eNodeB 101a is also connected via an X2 interface to a neighboring eNodeB 101b with a transmission point 102b serving another cell 105b. In an UTRAN, a Radio Network Controller (RNC) is connected to and controls a nodeB, and is, among other things, in charge of management of radio resources in cells for which the RNC is responsible. In UTRAN, the RNC is connected to the core network.
High Speed Packet Access (HSPA) is a mobile communication protocol that has been developed to cope with higher data rates in UTRAN. In the current HSPA standardization work, signaling load and overhead reduction is an important area. Furthermore, there is also standardization work ongoing for LTE on how to further improve Handover (HO) performance, where reduced signaling overhead is one important part.
High signaling load might be due to either frequent Radio Resource Control (RRC) state changes, or frequent HO signaling messages. An example of RRC state changes is when the UE switches between two different RRC states, such as between RRC_IDLE and RRC_CONNECTED states. A typical signaling or message exchange for the RRC state change in UTRAN is shown in the signaling diagram of FIG. 2, illustrating the signaling between the UE 20 and the RNC 25.
The signaling is typically initiated by a measurement report 201 transmitted by the UE 20 to the RNC 25, which comprises the status information of the UE side. The measurement report 201 is followed by a command message such as the RRC Reconfiguration message 202 from the network side, which comprises the setup information that is required to change the UE state. After the state change, the UE 20 will respond with a confirm message such as the Reconfiguration Confirm message 203. Unnecessarily frequent RRC state events may be avoided with well-tuned settings.
A typical HO signaling procedure in E-UTRAN is shown in the signaling diagram of FIG. 3, illustrating the signaling between the UE 30, a serving eNodeB 35, and a target eNodeB 37. First, the serving eNodeB 35 will send an RRC reconfiguration message 301 to the UE 30 in its cell, comprising measurement configuration information such as what neighbour cells to measure and HO parameter values defining when to trigger a HO. The actual HO procedure is initiated by a Measurement report message 302, which is transmitted from the UE 30 to the serving eNodeB 35 on the network side. The Measurement report message 302 is triggered based on measurement events which are performed periodically by the UE 30 in accordance with the RRC reconfiguration message 301. To limit the reporting frequency, HO parameters such as a hysteresis threshold and a time to trigger will be used. The Measurement report message 302 comprises a measurement list with radio channel quality measurements for a number of neighboring cells which are potential target cells. The serving eNodeB 35 makes a HO decision, i.e., selects a target cell for the HO among the potential target cells upon reception of the Measurement report message 302. When the serving eNodeB 35 has selected the target cell, it will via the X2 interface communicate with the target eNodeB 37 controlling the selected target cell. The serving eNodeB 35 sends a HO request 303 to the target eNodeB 37. If the target cell is suitable for receiving the UE after the HO, the serving eNodeB 35 will receive a HO request ACK message 304 from the target eNodeB 37, and will then transmit a HO command message 305 to the UE 30. Upon the reception of the HO command message 305, the UE 30 will access the target cell by the RACH channel by sending a RACH access message 306 to the target eNodeB 37. If the access is successful a RACH access ACK message 307 is returned to the UE 30 together with a resource allocation for the access. Finally, the UE 30 will send a HO confirm message 308 to the target eNodeB 37.
As illustrated in FIG. 3, a HO procedure is composed of a sequence of RRC signaling messages. Some of the messages are big in size, such as the Measurement report message 302 and the HO command message 305, which thus contribute more to the signaling load than other messages. Frequent HO signaling exchange may be difficult to avoid. Well balanced HO event triggering settings may decrease the numbers of HO events. However, minimizing HO signaling may negatively impact the user performance if it results in that the HO is initiated too late.
When the signaling load is high, network nodes such as the RNC in UTRAN or the eNodeB in E-UTRAN are highly loaded by the processing of the RRC signaling. Furthermore, RRC signaling transfer will typically be transmitted with absolute priority over other data traffic. Hence, a high signaling overhead consumes radio access network resources, which means that fewer resources are left for data traffic processing. The consequence of high signaling load is an increased possibility of dropped connections due to a long connection setup delay, and thus a degraded user perceived performance.